You're question isn't too clear. Are you saing that you have performed a full cPanel backups as configured in WHM and you want to restore only the mail files from that backup file, i.e. you have already created the account on the new server? If so, then you should open up the tarball and copy the mail directory over the top of the existing one and then fix the file ownerships to be correct.

Or, are you and end-user with a user cPanel backup? If so, then you need to speak to your web hosting provider.